The official Pokémon website has announced the format that will be used in all remaining Video Game Championships in the 2014 season. The major difference compared to previous seasons has to do with the pool of Pokémon the players can choose from. Participants in the 2014 VGC tournaments can only use Pokémon from the Kalos Pokédex (excluding Xerneas, Yveltal, Zygarde and Mewtwo). Also, those Pokémon must be Kalos native, that means that they must be caught or hatched in Pokémon X or Y only. Pokémon transfered to X/Y via Pokémon Bank and Poké Transporter are not legal in the VGC format.
For more information about the lesser rule changes, check out the document on the official site.